It is conventional in the production of polyamides, especially nylon, to subject the polyamide to treatment with a catalyst in order to increase the molecular weight of the polyamide, the process being termed "amidation". The general reaction involved in nylon amidation is the formation of an amide bond by the reaction of terminal carboxylic and amino groups of the polymer, with the process being generally conducted at an elevated temperature, particularly near the melting point of the polymer, in the presence of the catalyst. Such a process is described in U.S. Pat. No. 3,763,113 wherein the catalyst is a phosphonic acid of the formula: ##STR1## wherein n is 1, 2 or 3, and R.sup.1 is a mono-, di- or tri-valent organic radical depending on the value of n.
The patent states that the organic radicals represented by R, which are attached to the phosphorus atom through a carbon atom of said radical, are preferably mono-, di- or tri-valent aliphatic, cycloaliphatic, aryl, aralkyl or aralkenyl radicals, or such radicals in which a carbon atom is replaced by a hetero atom, in particular the nitrogen atom. It is most preferred that the organic radical be a lower alkyl, a cyclohexyl, a monocyclic aryl lower alkyl or a monocyclic aryl radical. Of all the examples in the patent, the only example of an organic radical wherein R contains a hetero atom is Example 8, employing 1-piperidinophosphonic acid.
In another patent by the same inventors, Burrows et al, U.S. Pat. No. 3,944,518, the combination of a phosphonic acid salt with an organic diamine is employed in order to increase the polymerization rate as well as to provide a polymer of improved whiteness. Mostly all of the examples utilize cyclohexylphosphonic acid, but in Example 12, part (d), there is an incidental experiment of .beta.-(ethylamino)-ethylphosphonic acid being employed in conjunction with hexamethylene diamine.
In French patent No. 1,531,145, corresponding to British 1,193,435, the use of alkali and alkaline earth metal salts of a phosphonic acid is disclosed to produce high molecular weight nylon-6,6 and 6,8. Among the acids used to make the salts is p-methoxyphenylphosphonic acid.
Also, in U.S. Pat. No. 3,509,107 phenylphosphonic acid is disclosed to increase the relative viscosity of nylon-6,6.
Finally, in U.S. Pat. No. 3,365,428, including the prior art cited therein, analogous phosphonic acids and salts thereof are disclosed, the patent specifically disclosing the use of such compounds as catalysts for the polymerization of polyamides. (Though not directly relevant, in U.S. Pat. No. 3,825,508, various reducing hydrocarbon-substituted phosphonic and phosphoric acids are proposed for use in the polymerization of a polyamide of a diamine and methyl tere- or iso-phthalic acid, for different purposes.)
The sum teachings of these references may be characterized as emphasizing by far those acids wherein R is a hydrocarbon and not motivating those skilled in the art to do further research with hetero substituted compounds in particular.
Though the specific catalysts employed in the precedingly discussed references increase the rate of the amidation reaction, still better catalysts are always desirable, especially catalysts which are stable, do not interfere with the antioxidants and obtain a linear polymer with as few side chains as possible.